The present invention relates to an electrochemical biosensor that can be used for the quantitation of a specific component (analyte) in a liquid sample. Electrochemical biosensors of the type under consideration are disclosed in U.S. Pat. Nos. 5,120,420 and 5,264,103. These devices have an insulating base upon which carbon electrodes are printed with the electrodes being covered with a reagent layer comprising a hydrophilic polymer in combination with an oxidoreductase specific for the analyte. These patents typically involve a spacer element, a generally U shaped piece and a cover piece, so that when the base, spacer element and cover piece are laminated together, there is created a capillary space containing the electrodes covered by the reagent layer. In addition to the oxidoreductase, there is included an electron acceptor on the reagent layer or in another layer within the capillary space. A hydrophilic polymer, e.g. carboxymethylcellulose, is used to facilitate the drawing of the aqueous test fluid into the capillary space.
In U.S. Pat. No. 5,141,868 there is disclosed another sensor in which the electrodes are contained within a capillary space. This reference describes the method of preparing a sensor by mating the base and cover plates which are adhered to the base to form a capillary space into which a fluid test sample such as blood is drawn. An alternative to this design is disclosed in U.S. Pat. No. 5,798,031 in which the sensor is comprised of two pieces, a base and a concave lid which, when fused together, form the capillary space. In either embodiment, working and counter electrodes are screen printed onto the base so that an electrochemically created current can flow when these electrodes are electrically connected and a potential created between them.
These devices have a base plate and lid which are laminated together with the U shaped spacer element in between so that the U shaped portion is open to provide a capillary space between the base and the cover. Touching the opening in the side of the sensor to a drop of test fluid such as blood results in the blood being drawn into the capillary space, so that it covers the reaction layer on the surface of the working electrode. An enzymatic reaction between the oxidoreductase creates a flow of electrons which are carried by a mediator such as ferricyanide to the working electrode and flow through the working electrode to a meter which measures the magnitude of the current flow. The counter electrode serves several purposes. First, it provides a fixed potential against which the working electrode is controlled. Second, for a two electrode system, such as that depicted in FIGS. 1 and 2, the counter electrode is used to complete the electrical circuit. In this mode, each electron that is transferred to the working electrode is returned to the test solution on the counter electrode side. The device's software is programmed to correlate the magnitude of this flow with the concentration of analyte in the test sample. In order for this current to flow, a complete circuit is formed by covering both electrodes with the conductive test fluid and applying a potential therebetween.
A problem which is sometimes associated with this sort of sensor occurs when an insufficient amount of blood is applied to the opening so that the counter and working electrodes are not completely covered with the sample, resulting in an incomplete current flowing across the electrodes. Since the amount of analyte such as glucose detected by the sensor is directly portional to the current flowing through the detection meter, failure to completely cover the sensor's electrodes can result in an artificially low reading of the blood sample's analyte, e.g. glucose concentration. One technique for dealing with this under filling problem is disclosed in U.S. Pat. No. 5,628,890 which involves a mechanism for preventing any response from being detected when the sample volume is too low to provide an accurate reading. This device involves a strip comprising an elongated electrode support defining a sample transfer path for directional flow of the sample from a sample application point. There is placed a working electrode in the sample transfer path and a counter or reference electrode down stream from the working electrode in the sample transfer path. Failure of the blood sample to totally cover the working electrode will result in no response from the reading mechanism due to the absence of a closed circuit through which current can flow. Another technique for detecting short fills is disclosed in U.S. Pat. No. 5,582,697 where there is described a third electrode located downstream from the working and counter electrode, so that the circuit between the three electrodes will not be completed in the event of a short fill.
It would be desirable and it is an object of the present invention to provide an electrochemical sensor which affirmatively notifies the user when insufficient sample has contacted the electrodes. Upon receiving such a notice the user knows that an accurate reading cannot be obtained and that the sensor should be discarded in favor of a new one.